The embodiments described herein relate generally to imaging objects, and more particularly, to generating two-dimensional images of objects from projection data acquired at multiple views.
In some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at each detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray fan beam intersects the object constantly changes. A group of x-ray attenuation measurements (e.g., projection data), from the detector array at one gantry angle may be referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one or more revolutions of the x-ray source and detector about the object or patient being imaged.
Many modern CT systems are helical scanners (also known as spiral scanners), in which the scanned object is continually moved while the projection data is being acquired. The path of the x-ray source describes a helix with respect to the scanned object. Most helical scanners have multiple rows of detectors, and the x-ray fan is collimated into a cone to illuminate the entire array of detectors. The angle between the x-ray source and the first and last detector rows is referred to as the “cone angle”.
The entire scanned volume scanned by the helical scanner can be reconstructed using well known tomographic reconstruction algorithms such as direct Fourier or filtered back projection methods. Many of these techniques require a relatively large amount of computation.
Orthographic and SP-like images can be created from the reconstructed volumetric data by projecting digitally through the reconstructed data. This requires significant additional computation, and the resulting images may have relatively poor resolution, making it difficult to distinguish features of the imaged object.